a) Field of the Invention
The invention is directed to a method for determining the distance of a scanning probe of a scanning probe microscope from a specimen surface to be examined, wherein the near-field probe is excited to oscillations lateral to a specimen surface to be examined and at least one amplitude signal and/or frequency signal of the oscillating scanning probe is recorded.
In addition to the method for determining distance, the invention also provides a method for imaging the topography of a specimen surface and a scanning probe microscope, in particular an optical scanning probe microscope, which comprises at least one holder for a specimen surface to be examined and at least one device for exciting oscillating movements of the scanning probe at least in the plane of the specimen surface to be examined.
b) Description of the Related Art
The scanning probe microscopy family includes, for example, near-field microscopy (abbreviated SNOM: Scanning Near-field Optical Microscopy), scanning tunneling microscopy (STM), force microscopy (SFM), and scanning electrochemical microscopy (SECM). The method according to the present invention will be described more fully hereinafter with reference to near-field microscopy, although it should not be considered restricted thereto. It will be a simple matter for the person skilled in the art to transfer the general principles to different types of scanning probe microscopy. Optical near-field microscopy makes it possible to overcome the conventional diffraction-limited resolution limit and to examine structures that are far smaller than half of the wavelength of the utilized light.
For operation of an optical near-field microscope, a small aperture must be brought to within a few nanometers of the specimen surface to be examined. Light emitted from this aperture interacts with the specimen and can then be evaluated by means of suitable collection optics and a detector. A reversal of the light path (i.e., external illumination, collection of the light by the fiber) is also possible. An image is generated in that the probe (aperture) is moved over the surface point by point and line by line and the data obtained in so doing are evaluated electronically and compiled to form an image.
A type of probe used in near-field optics is a tapered glass fiber which is produced by etching and/or drawing and is coated with metal in such a way that only the area of the fibers at the very front remains permeable to light (aperture). With respect to these probes, reference is had to E. Betzig, J. K. Trautmann, T. D. Harris, J. S. Weiner and R. L. Kostelak, Science 257, 1468-1470 (1991), wherein the disclosure contained in this publication is incorporated in its entirety in the present Application.
The shear force detection method, for example, is used to determine the topography of the specimen surface and the distance between the probe and surface and to maintain regulation thereof constant during measurement. In this regard, reference is had to E. Betzig, P. L. Finn and J. S. Weiner, Appl. Phys. Lett. 60, 2484-2486 (1992) and R. Toledo-Crow, P. C. Yan, Y. Chen and M. Vaez-Iravani, Appl. Phys. Lett. 60, 2957-2959 (1992). The disclosure contained in these publications is incorporated in its entirety in the present Application.
In the shear force detection method, the glass fiber probe which is situated almost vertical to the surface is set in mechanical resonance, so that it executes lateral oscillations, i.e., oscillations parallel to the surface. The frequency range of the resonance is preferably between 10 kHz and 4000 kHz, depending on tip geometry. As the glass fiber tip approaches the surface, there occur at a distance of a few nanometers, preferably less than 30 nm, shear forces which act between the tip and the specimen and which cause a change in the amplitude and phase of the oscillation. The detected change in amplitude and/or frequency and/or phase can be used to regulate the distance between the probe and the surface or to determine the topography of the specimen surface.
Different detection methods have been developed for measuring the amplitude, frequency or phase of the tip oscillations; these methods can be distinguished as either optical or non-optical. With respect to optical detection methods, reference is had to E. Betzig, P. L. Finn and J. S. Weiner, Appl. Phys. Lett. 60, 2484-2486 (1992); R. Toledo-Crow, P. C. Young, Y. Chen and M. Vaez-Iravani, AppL. Phys. Lett. 60, 2957-2959 (1992); and R. D. Grober, T. D. Harris, J. K. Trautman, and E. Betzig, Rev. Sci. Instrum. 65, 626-631 (1994). For non-optical methods, reference is had to J. W. P. Hsu, Mark Lee and B. S. Deaver, Rev. Sci. Instrum. 66, 3177-3181 (1995); K. Karrai and R. D. Grober, AppL. Phys. Lett. 66, 1842-1844 (1995); J. Barenz, O. Hollricher and O. Marti, Rev. Sci. Instrum. 67, 1912-1916 (1996); R. Brunner, A. Bietsch, O. Hollricher, and O. Marti, Rev. Sci. Instrum. 68, 1769-1772 (1997). The disclosure contained in these publications is incorporated in its entirety in the present Application.
It has been shown that a drop in amplitude first appears at a distance of a few nanometers from the specimen, so that an interaction between the probe and surface that can be regulated first occurs in this range.
In order to obtain the topography of near-field optical imaging, regulation at constant shear force has always been used up to this point, i.e., the probe was located in the interaction range with respect to the surface during the entire duration of recording and measurement. Alternatively, in order to eliminate electrochemical reactions on the specimen surface, it was suggested that a constant distance be maintained between the specimen and probe tip. In this connection, reference is had to P. I. James, L. F. Garfias-Mesias, P. I. Moyer, W. H. Smyre, xe2x80x9cScanning Electrochemical Microscopy with Simultaneous Independent Topographyxe2x80x9d, J. Electrochem. Soc. vol. 145, no. 4, pp L-64-L66, wherein the disclosure contained in this publication is incorporated in its entirety in the present Application.
Due to the fact that the probe is located in the range of interaction with respect to the surface during the entire measurement in the measuring and imaging process according to the prior art, the probe and specimen are also loaded by shear forces during the entire duration of recording. With soft specimens, this continual loading during measurement can lead to deformation or even complete destruction of the specimen or specimen surface. Aside from destruction of the specimen surface, the probe can also be damaged or rendered unusable due to the constant loading. These problems occur in particular when examining organic specimens, for example, soft polymers and biological specimens. In extreme cases, because of the problems described above, it is impossible to image the surfaces and topography of soft specimens of this kind.
Accordingly, it is the primary object of the invention to provide a method and a device by which the disadvantages described above can be eliminated. In particular, an imaging method will be provided which also allows examination of soft specimens, especially the topography thereof, as well as a corresponding scanning probe microscope, especially an optical near-field microscope.
According to the invention, this object is met in a method for determining the distance of the scanning probe in that an oscillating movement of the scanning probe and specimen to be examined relative to one another is superimposed on the lateral oscillation of the scanning probe. The distance of the scanning probe from the specimen is then determined from the amplitude signals and/or frequency signals and/or phase signals obtained from the movements of the scanning probe.
In a first embodiment form of the invention, the specimen surface is stationary and the scanning probe oscillates vertically as well as laterally.
Alternatively, the scanning probe can be fixed vertically and the specimen surface oscillates vertically, for example, in that the holder on which the specimen is arranged is set in oscillation.
In an advantageous embodiment form, the vertically oscillating movement has a sinusoidal shape.
The amplitude of the vertically oscillating movement is preferably in the range of 10 nm to 1 xcexcm and the frequency is in the range of 10 Hz to 10 kHz.
In a particularly preferable manner, the invention is applied in optical near-field microscopy without being limited thereto. With respect to the near-field tips in optical near-field microscopy, reference is had to J. Barenz, O. Hollricher and O. Marti, xe2x80x9cAn Easy-to-use Non-optical Shear-force Distance Control for Near-field Optical Microscopesxe2x80x9d, Rev. Sci. Instrum. 67 (5), (May 1996), pages 1912-1916, wherein the disclosure contained in this publication is incorporated in its entirety in the present Application.
In addition to the method for determining distance, the invention also provides a distance regulating method for a scanning probe microscope in which the distance between the probe and the specimen surface is determined in accordance with a method described above and the scanning probe is advanced toward the specimen until reaching a predetermined damping value and/or frequency value and is then moved away from it.
The method described above can also be used with appropriate evaluating electronics to image surface topographies. A method of this kind can include the following steps, for example:
First, a damping value S0 of the laterally excited oscillation is determined at a starting point (0, 0) of the specimen for a distance d0 (0, 0) of the specimen probe from the specimen surface without vertically oscillating movement of the specimen probe;
the specimen probe is excited to vertically oscillating movement and the specimen surface is scanned point by point (x, y);
the damping ST of the specimen probe is determined for every specimen point (x, y) as the vertically oscillating movement traverses a predetermined specimen distance dT;
when S greater than S0, the distance d0 (x, y) between the specimen probe and specimen surface is increased;
when ST=S0, the distance d0 (x, y) is maintained constant, so that d0 (x, y), which is determined in this way, reproduces the surface topography of the specimen.
The invention also provides an optical scanning probe microscope which has a scanning probe, at least one holder for a specimen surface to be examined, and at least one device for exciting oscillating movements of the scanning probe in the plane of the specimen surface to be examined (lateral oscillations) as well as vertical thereto (vertical oscillations).
In a particularly preferred manner, the scanning probe microscope has at least one piezo element for exciting the vertically oscillating movement. It is possible to excite the topography balance or topography compensation as well as the oscillation vertical to the specimen surface to be examined with only one piezo element.
In an improved, further-developed embodiment form, the periodic vertical modulation and topography compensation can be carried out by means of two separate piezo systems.
In order to be able to regulate and/or control the distance between the specimen probe and the specimen surface, it is provided in a further development of the invention that an amplitude damping of the lateral oscillation is predetermined, for example, by means of an electronic circuit, wherein the periodic change in the distance between the probe and surface may not exceed this amplitude damping. The laterally oscillating probe and the specimen then come closer to one another periodically until a predetermined amplitude damping is reached. Subsequently, they move away from each other again.